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Fig. 17.3. Micrograph of Gos-Impregnated Graphite Specimen, GI-5. Graphite NCC R0025. Impregnation,
2O1Z0F/6 hr/C3H6. Bright field. 750~.
diameter, which suggested a buildup of a surface
layer. Metallographic examination indeed re-
vealed a surface buildup of approximately 15 p.
We found it difficult to resolve pyrocarbon deposits
inside the graphite pores by opt
but in isolated areas (see Fig. 17.3), there was
e of a buildup of pyrocarbon in in-
We machined approximately 4 mils
from the specimen surface and found that the per-
meability increased (K > 10 --5 cm'Jsec) rapidly,
which indicates that the low-permeability layer is
n. At present we are optimizing our depo-
sition conditions to incre e the depth of the impervious
layer.
We are also preparing irradiation speci
future HFIR experiment. Although our preliminary
results are encouraging, the feasibility of the technique
can only be assessed by fast-flux irradiation
experiments. Because of the dimensional instability
of graphite when irradiated, we must determine
whether the low eability of the graphite
will be maintained or whether the pyrocarbon will
change dimensions at a different rate and the permeability
increase.